1. The effect of lithium treatment on the leukocyte, liver and muscle glycogen levels of guinea-pigs has been studied. 2. A 4-week treatment with lithium chloride (5 mmol/l i.p. in saline) increased leukocyte, liver and muscle glycogen and decreased blood glucose levels. 3. These results strongly suggest that a lithium-induced insulin-like effect may be related to the long term effect.